1. Field of the Invention
This invention relates primarily to dynamoelectric machinery utilizing a superconductive field winding, and more specifically, this invention relates to a dynamoelectric machine utilizing a superconductive field winding that can operate effectively as an alternating current (AC) motor in both the synchronous and asynchronous modes.
2. Description of the Prior Art
Within the last ten or twelve years, considerable interest has arisen in utilizing the extremely low electrical resistance state that certain materials exhibit at cryogenic temperatures, known as superconductivity, in the construction of dynamoelectric machinery. As a result of the extremely high magnetic fields that can be produced by utilization of the superconductive effect, prior art efforts in the superconductive machinery field have been directed toward the utilization of an essentially all air gap structure in which ferromagnetic material is not utilized. Such efforts have produced dynamoelectric machines that operate reasonably well as synchronous AC motors. However, these machines do not operate effectively during the asynchronous mode of operation encountered during starting or in the presence of transient conditions.
One of the difficulties encountered when an all air gap construction is utilized for an AC motor is that the torque produced during asynchronous operation is relatively small. This results from the fact that during starting the machine operates as an induction motor with a conventional strength (or even smaller) AC magnetic field. Since the conventional strength field has a magnetic flux density that is only a small fraction of that produced by a superconductive winding, the lack of a ferromagnetic path results in very inefficient utilization of the relatively small amount of flux available. Accordingly, the torque produced during this phase is relatively quite small.
Another problem that arises during asynchronous operation of a superconductive AC motor is that relatively low frequency AC magnetic fields, such as those produced at slightly subsynchronous speeds, penetrate into the superconductive winding. Since superconductivity is essentially a DC phenomenon, the introduction of AC fields results in relatively high energy losses that are unacceptable. Not only are the losses undesirable; they may result in "quenching" or loss of the superconductive effort. Thus, prior art devices have had to include a separate flux shield, such as that disclosed in U.S. Pat. No. 3,679,920, MacNab et al, issued July 25, 1972.